This invention relates to a method based on solid modeling for automatically generating tool paths to achieve more efficient milling.
Efficient 21/2-axis milling is the key to fast production and cost reduction in the milling of molds and dies. In this type of machining the z depth is set and all tool motion is in the x and y planes, another z depth is set and so on. For any one set of tool paths, z is constant. Efficiency requires accurate tool paths, a minimized number of tool retractions, and automation with flexibility. Solid modeling systems can define parts to be milled, and milling machines can follow programmed paths faithfully, but the step from a solid model to an optimized process for automatically generating the numerical control (NC) code has been lacking and is addressed by this invention.
A solid model of a part to be machined may be constructed in a computer using the principles of constructive solid geometry by adding and subtracting a basic set of primitive shapes; this is explained in U.S. Pat. No. 4,618,924 - J. K. Hinds. After the model is defined by means of boolean operations on solids, the stock from which the model is to be milled, and avoidance and containment regions must be modeled. A typical avoidance region is a clamp to hold the workpiece, which can push against the sides of a workpiece or can clamp over the top of it. In either case, the centerline of a milling tool cannot move along the edge of a workpiece in the region of the clamp without milling the clamp. Containment regions may be needed when milling complex surfaces where it is necessary to generate one setup, mill a portion of the part, change the setup and mill a portion, etc. For each setup only a portion of the part is milled. Such an approach cannot be accomplished unless the system allows the user to define containment regions for each setup. Having the conglomerate model, part plus stock plus avoidance and containment regions, maximum and minimum z levels are identified from the conglomerate, and planar sections are passed through the workpiece from the maximum to the minimum level as discussed by Hinds. Tool paths are defined for each planar section, and the aggregate of these tool paths define the motion for the tool in the milling process. Thus the central problem to be solved involves the definition of optimal tool paths and retraction requirements on a single plane.
It is a simple matter to accumulate the optimized results as they are obtained on successive planes. The conglomerate defines the total milling process, which can be used with any milling machine. Machine code for a specific milling machine is generated in a post processing phase since each machine requires its own code format.
Tool paths are curves that describe the route for the centerline of the tool. Since the tool has a diameter, at all times these centerlines must maintain a minimum distance of the tool radius from the boundaries. In addition to the tool radius (HRAD), the user must be able to specify the distance between tool paths (STEPOVER) and extra thickness desired on the finished boundaries (THICK). All tool paths generated must honor these distance requirements within a fine tolerance.
The NC optimization process being described defines the generation of gouge-free tool paths on a single plane, optimizes the ordering of tool paths, and specifies required retractions. It optimizes the process of milling cavities which define the mold or die for forging parts. All tool paths are contained within the cavity. The workpiece surrounds the cavity and is of size and shape suitable for the purposes of the forging operation, but is not of concern for this invention. Hence the workpiece is shown only once and as a rectangular slab in the figures.
A consideration is that the cavity to be milled is of arbitrary shape and may have deep, concave portions and/or narrow strictures. As the tool paths are generated, major challenges arise with maintaining data concerning the position of each tool path relative to the others to determine accessibility of the tool from one path to another without gouging. The technique developed involves establishing algorithmic rules to nest, or group, certain sets of tool paths and then deriving another set of algorithmic processes to mill these nests in an expedicious manner while honoring milling guidelines and minimizing the number of required retractions. These algorithmic processes are the essence of the invention.
Constant interference checking is mandatory to prevent gouging of the boundaries. A problem in the development of this process is the handling of avoidance and containment regions. Another consideration is that pocketing tool paths, which are closed loop tool paths to clear material inside cavity boundaries, are generated for flat-end cutters only. Paths are also appropriate for ball-end cutters when the curve is a section cut of an offset surface. Thus the optimized system needs a mechanism for knowing when the part is an offset surface.
Finishing curves are tool paths closest to cavity boundaries. Quality molds require smooth boundaries, which puts constraints on the manner in which a cutting tool approaches and leaves finishing paths. Predrilling and straight line motion from neighboring arcs can cause dwell marks where the tool is deflected and seats in a location before traversing a boundary. To prevent this, the process should augment all finishing curves with tangential approach and exit arcs, and determine that there is sufficient room for the cutter tool diameter at all points of the augmented curve.